1. Field of the Invention
The present invention relates to a nuclear reactor, and more particularly to a nuclear reactor that allows for equalization and/or optimization of power distribution in a reactor core.
2. Description of the Related Art
Pebble bed reactors (PBR) have a flow path (fuel region) that is surrounded by a reflector(s). Sphere-shaped fuel bodies (fuel pebbles) containing nuclear fuel material undergo nuclear reaction inside the flow path thereby producing power. In view of safety, it is preferable for the nuclear reactors in general to have equalized power distribution in a reactor core. In the PBR, the fuel body goes down the flow path and is repeatedly removed and reloaded, whereby the power distribution in the nuclear reactor is equalized in an axial direction (i.e., direction in which the fuel bodies flow in the flow path). Hence, while the nuclear reactor is running, replacement (removal and reloading) of the fuel bodies is continuously performed, i.e., removed fuel bodies are inspected, and new fuel bodies are loaded.
For example, a 170,000 kilowatts (kW) class nuclear reactor uses 415,000 fuel bodies, among which 6,000 are removed from the nuclear reactor for inspection every day. Among the removed 6,000 fuel bodies, approximately 600 are replaced with new ones. New fuel bodies are loaded into the nuclear reactor together with the approximately 5,400 remaining fuel bodies. Generally, average life of the fuel body is about two years, and one fuel body is reloaded nine times on average during its life.
However, if the replacement of the fuel bodies becomes more frequent, operational costs (costs for inspection and replenishment of the fuel bodies) increase accordingly, which in turn results in an increased cost for power generation. Hence, it is preferable to equalize the power distribution in the nuclear reactor without increasing the frequency of fuel replacement.
One conventional technique applicable to the pebble bed reactor is known from Japanese Patent Application Laid-Open No. 2003-222693. The conventional reactor (nuclear reactor facility) described therein includes a detecting unit that receives plural fuel spheres discharged from a reactor core and detects burnup of the fuel spheres, and a sorting unit that determines a radial loading position from which the fuel sphere is reloaded into the reactor core according to the result of detection by the detecting unit.
In a block reactor, a fuel body having a hexagonal columnar shape houses fuel compacts. Base material of the fuel compact is graphite. The fuel compact is filled with coated fuel particles (of approximately 1-mm diameter). Nuclear fuel material contained in the fuel particle undergoes nuclear reaction thereby producing power. For the equalization or the optimization of the power distribution in an axial direction, enriched uranium is prepared in twelve different levels of enrichment, for example. Preparation or manufacture of enriched uranium at various levels of enrichment in a large amount pushes up the manufacturing cost of the fuel.